1. Field of the Invention
The present invention relates to a technique for controlling a valve timing of a diesel engine equipped with a common-rail fuel injection device, more specifically, a technique for controlling a valve timing suitable for a common-rail diesel engine, provided with a fuel injection device including an injector having a plurality of intersections between an axis line of the injector and the axes of injection holes bored in a nozzle body of the injector, or a group of injection holes.
2. Related Art
Conventionally, there is a well-known common-rail fuel injection device provided with a diesel engine. There is also a well-known concrete configuration of an injector injecting fuels supplied from the common-rail.
Moreover, an injector, which has a plurality of intersections between an axis line of the injector and the axes of injection holes bored in a nozzle body of the injector, i.e., which has a group of injection holes, is heretofore known.
The injector having a group of injection holes can have smaller injection holes than an injector having the same amount of injection quantity without a group of injection holes. Therefore, it is noted that the injector having a group of injection holes enables injected fuels to be atomized and diffused over a wider range, compared to the injector without a group of injection holes, thereby improving an ignition performance and securing a low combustion noise and good combustion state, even when the engine is cool and low cetane value fuels are used.
It is also known that the injector having a group of injection holes is effective as a means for cleaning up an exhaust gas since it enables total hydrocarbons (THC) and particulate matters (PM) contained in the exhaust gas to be reduced. For example, JP 2006-70802 discloses the above-mentioned technique.
However, it is proved from experimental results that nitrogen oxides (NOx) or smokes in the exhaust gas are increased, due to increases of intake temperature and fuel temperature, or an increase to a high cetane value of the fuels.
Consequently, when so-called intake temperature correcting control is performed, a range of reduction in fuel injection quantity must be increased, with increase in the intake temperature, thereby lowering torque and causing a problem in engine performance.
Although it is effective to lower the intake temperature so as to inhibit NOx or smokes, there is a limitation of cooling capacity in an air system, and there is a limitation of cooling capacity, depending on an intercooler capacity even in a water-cooling system. Considering tighter control on exhaust emissions in the future, measures to increase the capacity of the intercooler may be taken, but additional measures exist to inhibit NOx or smokes, since service conditions thereof remain to be limited due to the limitation of installation location and cost phase thereof.
In other words, in a conventional art, it was difficult to put to practical use an engine equipped with an injector, which reaches the environmental standard value (regulation value) and has a group of injection holes.
Technologies that reduce NOx by delaying a peak of combustion so as to lower the combustion temperature by means of injection timing delaying are well-known, but they effect fuel consumption due to the deterioration of combustion efficiency, whereby there still remain problems/limitations such as limitation of delaying or the like.
Previously, as an effective means for lowering the intake temperature, a technology which controls valve timing of the engine and delays a timing of closing the air intake valve so as to drive an engine through a high expansion ratio cycle (so-called miller cycle), in which the expansion ratio is higher than a compression ratio, has been heretofore known.
It is known that when the engine is driven in the miller cycle, an intake temperature in a cylinder can be lowered and a combustion temperature can be lowered, due to the effect of intake air adiabatic expansion. For example, JP 2004-360459 discloses this technology.